Generate large numbers for interval

I need to randomly produce two numbers L and R such that 1 ≤ L ≤ R ≤ 10^18. How this can be done ? Please help.

My Code :

#define MAXN 1000
int main(){
    srand(time(NULL));
    int x=rand()%MAXN;
    int y=rand()%100000;
    int z=rand()%100000;
    long L =x; 
    long R=x*y*z;
    assert(L<=R && L>=1 && L<=1000000000000000000 && R>=1 && R<=1000000000000000000); 
    cout<<L<<" "<<R<<"\n";
}

Obviously its not right way. So i am looking for best way to do it.